1. Field of the Invention
The present invention relates generally to methods and circuits for driving light emitting diodes, and more specifically to methods and circuits for driving light emitting diodes to extend battery life.
2. Description of the Prior Art
The use of light emitting diodes (LEDs) has become increasingly popular in small, portable, battery-powered electronics. For example, many handheld electronics incorporate color displays that use white LEDs as a backlight. Many of these LEDs require a drive voltage that is higher than the voltage of the battery pack power source. For example, the forward voltage drop of a white LED may be approximately 3.5 volts, which is a voltage higher than a device powered by one or two cells can provide.
In order to provide the high forward-voltage requirement of the LEDs and to regulate the drive current, specialized power converters for regulating or stepping-up voltage have been developed. Such power converters have been designed to minimize LED intensity variations with battery voltage, and to minimize brightness variations between different LEDs, which may be used, for example, to light portions of the same color display.
Most of the specialized power converters fall into one of two commonly used regulator types: inductor-based boost converters and capacitor-based charge pump converters.
The boost converter works cyclically by storing energy in an inductor when a switch is on, and dumping the stored energy together with energy from the input into the load when the switch is off. The output voltage is controlled and regulated by varying the amount of energy stored and dumped each cycle. When the switch is on, the supply voltage is applied across the inductor, and the current through the inductor increases linearly. During the on state, the capacitor supplies the load with energy and, thus, the voltage across the capacitor is reduced. When the switch is turned off, the current continues through the inductor, supplying the load via a diode. Consequently the current decreases linearly.
A charge pump uses two or more capacitors and switches to charge and transfer charge from one capacitor to another, thereby producing an output voltage greater than the input voltage.
In battery powered applications, such power converter circuits are typically designed to maintain a constant current through one or more LEDs to maintain constant LED brightness over the entire range of battery voltages, from full charge to almost fully discharged. While it may be aesthetically pleasing, attempting to maintain full drive current as the battery discharges greatly reduces the duration of battery powered operation, particularly near the end of the battery's charge. Many times the operator of a battery powered device would rather operate with dimmed LEDs for a longer period of time rather than with fully bright LEDs for a shorter period.
Therefore, there is a need for a method and circuit for regulating the current through an LED while taking into account battery voltage and extending the useful battery life.